Natural and recombinant products are in growing demand for research and therapy. Large scale production of such cell products has traditionally tended to the use of larger vessels for more production. See, for example, Arathoon, W. R., et al., Science 232:1390 (1986). Whereas large vessels may be satisfactory for bacterial fermentations, such vessels do not provide adequate circulation or nutrients, etc. necessary for the growth of eucaryotic cells, such as mammalian or plant cells.
To increase product yields, various immobilization procedures used in bioreactors have been developed. Immobilization procedures increase the cell density of the culture, which results in increased productivity and, therefore, smaller volume bioreactors may be used. Various immobilization procedures include the carrier-binding method, the cross-linking method and the entrapping method. The carrier-binding method is based on direct binding of cells to water-insoluble carriers by physical adsorption, ionic and/or covalent bonds. Cells may also be immobilized by cross-linking each other with bi- or multi-functional reagents. Finally, a method of entrapping cells into polymer matrices has been developed.
The various types of immobilization procedures may be used in bioreactors such as airlift bioreactors known in the art. See, for example, "Airlift Fermentors: Construction Behavior and Uses", Advances in Biotechnology Processes I, pp 67-95, Alan R. Liss, New York, N.Y. (1983).
A particular commercially available airlift bioreactor which can be readily modified according to the present invention has an upper growth chamber which is connected to a lower smaller diameter mixing chamber via a downwardly and inwardly sloping conical section. A gas mixture is sparged into the mixing chamber and sets up a gentle circulation of liquid growth medium within the growth chamber. This bioreactor accomodates immobilization procedures for both suspension cells and for attachment dependent cells.
Anchorage dependent cells are those cells which must be grown in an environment wherein they can be attached to a support material. This attachment has been accomplished, for example, by attaching cells to stacked petrie dishes or by growing the cells in roller bottles. Each of these approaches has drawbacks such as the inability to achieve high volumetric cell density or inadequate circulation of nutrients, etc. necessary to grow the cells.
Certain cell lines are suspension cells as is well known in the art. This means that the cells must be grown in an environment where they are suspended in a liquid growth medium and circulated to provide adequate contact with nutrients and dissolved gases to promote cell growth. Suspension cells, such as hybridomas, may be immobilized by entrapment in polymer matrices in the form of beads which are then circulated within the liquid growth medium in the bioreactor.